This invention relates to a marine propulsion system, and more particularly to a stern drive marine propulsion system having a mechanism for varying the height of the propeller relative to the boat.
In marine drives of conventional design, the propeller is maintained at a fixed elevation below the bottom of the boat. With such a fixed height of the propeller, engine and propeller design must strike a compromise between desirable top speed at full throttle and desirable acceleration characteristics. A fixed height propeller can also cause difficulties in low speed shallow water operation by forcing the aft end of the boat downward toward the bottom of the body of water, thus increasing the likelihood of the propeller encountering an obstruction.
The present invention is designed to minimize or eliminate the design compromise which must be reached between desirable top speed and acceleration characteristics caused by a fixed height propeller. The invention also overcomes problems encountered when running in shallow water. In accordance with the invention, a marine propulsion mechanism includes a drive shaft rotatably driven by an engine and a transmission adapted to be driven by the drive shaft. The propeller is interconnected with the transmission and is rotatably driven thereby, and variable height means is interconnected with the transmission for varying the height of the propeller relative to the boat. The variable height means allows the drive shaft to maintain a fixed position relative to the boat, and preferably is disposed between the drive shaft and the transmission. In one embodiment, the variable height means includes a stationary housing fixed to the boat and a movable housing to which the propeller is connected. The movable housing moves upward and downward relative to the stationary housing and the height of the propeller relative to the boat is determined by the position of the movable housing. The variable height means preferably provides operation of the transmission along the entire range of movement of the movable housing relative to the stationary housing. A drive means is provided for moving the movable housing relative to the stationary housing. In one embodiment, a rotary input member is connected to the drive shaft and a rotary output member is connected to the transmission shaft. A rotary idler member is disposed between and engageable with the rotary input and output members. The rotary output member is interconnected with a fixed member provided on the movable housing. The drive means includes a rotatable wheel to which a shaft extending through the idler member is fixed. Rotation of the rotatable wheel causes the idler member to rotate about the rotary input member. A link or the like is connected between the rotary idler member and the rotary output member to maintain the center-to-center relation between such members. In this manner, rotation of the rotary wheel and movement of the idler member about the input member causes upward or downward movement of the rotary output member, and thereby upward or downward movement of the movable housing.